1. Field of the Invention
The present invention relates to an overcurrent protection circuit for protecting components in equipment containing a switching mode power supply from overcurrent due to a short-circuit.
2. Description of the Related Art
Generally, a switching mode power supply has been widely used as a main power supply in small electric and electronic equipment. The switching mode power supply device is small in size, light in its weight, high in efficiency and stable in output voltage.
A switching mode supply normally includes a power input unit for inputting external commercial AC power, a rectifier for rectifying the AC power inputting by the power input unit into DC power, a switching transformer for transforming an output voltage from the rectifier into voltages of various levels using a plurality of windings with different turn ratios and supplying the transformed voltages to components in electric or electronic equipment, a switching output unit for outputting a switching control signal to the switching transformer, a count detector for detecting the output current from the switching output unit, and error detection/feedback circuit for receiving a desired one of the output voltages from the switching transformer and for detecting a voltage error corresponding to the received voltage, a pulse width modulation circuit for generating a pulse width modulation signal with a desired duty cycle ratio in response to an output signal from the current detector and an output signal from the error detection/feedback circuit and for outputting the generated pulse width modulation signal to the switching output unit to control the switching time thereof, an initial drive voltage supply circuit for supplying an initial drive voltage to the pulse width modulation circuit and an initial drive voltage blocking circuit for blocking the initial drive voltage from the initial drive voltage supply circuit to the pulse width modulation circuit when the switching mode power supply begins to operate.
Initially, the output voltage from the rectifier is supplied to the drive voltage input terminal of the pulse width modulation circuit through the initial drive voltage supply circuit.
The pulse width modulation circuit outputs the pulse width modulation signal with the desired duty cycle ratio to the switching output unit which outputs the switching control signal to the switching transformer. The switching transformer outputs the voltages to the components of the equipment.
When the switching mode power supply begins to operate, the initial drive voltage blocking circuit prevents the initial drive voltage from being supplied to the pulse width modulation circuit.
If the switching transformer is short circuited, the amount of the output current therefrom is abruptly increased resulting in a reduction in the output voltages.
As a result, the drive voltage from the switching transformer to the pulse width modulation circuit is lowered below a value capable of driving the pulse width modulation circuit, thereby causing the pulse width modulation circuit to stop.
Since the pulse width modulation circuit then outputs no pulse width modulation signal, the switching output unit outputs no switching control signal to the switching transformer, thereby stopping the output of drive voltages to the components in the equipment.
However, a time delay is present from the short circuit of the switching transformer until there is a reduction of the drive voltage to the pulse width modulation circuit due to the electrostatic capacity of various elements in the switching transformer.
The pulse width modulation circuit is therefore still driven during the time delay, thereby causing the switching output unit to output the switching control signal to the switching transformer. As a result, the switching transformer supplies an overcurrent to the short circuit, resulting in damage to the equipment components.